Pipe couplings for joining two sections of pipe are well known. Such pipe couplings generally comprise a cylindrical body for accepting and retaining an end of each pipe section, an end connector attached to each end of the body by threads and a gasket disposed between the body and each pipe section. Each gasket is retained by an end connector.
In a typical prior art pipe coupling, axial movement of the body with respect to the pipe sections relies upon friction between the gaskets and the exterior surface of the pipe sections. The gaskets can be made from a material having a high coefficient of friction (such as rubber) or can be designed with a taper that tightens down against the pipe. Gasket friction, however, is relatively weak, so that such pipe couplings cannot resist strong axial movements.
In some applications, additional axial movement restraint is provided by a gripper washer disposed between each gasket retaining ring and its associated end connector. A gripper washer is a washer-like structure, typically a metallic structure, whose inner periphery comprises a large plurality of gripping fingers. The griping fingers are bent inwardly at an angle. The interior diameter of the gripper washer is chosen be slightly smaller than the outside diameter of the pipe sections. In prior art pipe couplings employing gripper washers, the outer periphery of each gripping washer is retained by the inner surface of an end connector. When a pipe section is thrust into one end of the body, the pipe section slips smoothly along the gripping fingers of the gripper washer. Once fully disposed within the body, however, the pipe section cannot be slid in a reverse direction relative to the gripping fingers. Thus, the gripping fingers retain the pipe section firmly to the body.
A problem arises when a pipe coupling relying on gripper washers is made from CPVC or other material which is “notch sensitive.” Notch sensitivity means that small cuts in the surface of the part can propagate into large cracks as a result of stress applied to the part. Notch sensitivity is a general term typically measured as the reduction of impact strength as determined by Izod impact testing (ASTM D256). Notches, however, also contribute significantly to fatigue failures in many materials, metals and plastics included. When a pipe coupling relying on gripper washers is made from a notch-sensitive material, the narrow outer peripheral edge of the gripper washer continuously contacts the inner surface of an end connector, thereby tending to cause the rapid failure of the end connector. If the end connector were to fracture, the pipe would come out of the body resulting in catastrophic failure.
Accordingly, there is a need for an improved gripper washer-containing pipe coupling which avoids this problem in the prior art. The present invention satisfies this need.